No practical solid state laser sources operate directly in the blue-green spectral region. Solid state lasers that can be pumped with efficient high power laser diode arrays are especially attractive for a wide variety of applications, such as communications and remote sensing, due to their compact size and weight and long lifetime. Neodymium based solid state lasers using diode pumps have been demonstrated with electrical to optical efficiencies of .about.10% in the high power Q-switch mode. However, although these lasers emit in the infrared near 1 micron, a desirability is unrealized to be able to shift the output to the blue-green spectral region in order to take advantage of the increased transmission in seawater. In addition, there are certain narrow wavelength bands throughout this region, the Fraunhofer lines, where the solar irradiance is significantly decreased; see the article by J. M. Beckers, C. A. Bridges, and L. B. Gilliam, report AFGL-TR-76-0126, Air Force Geophysics Lab., Hanscom AFB (1976). The H-Beta
Fraunhofer line at 486.13 nm is an especially attractive wavelength to operate a laser/receiver system due to its large width (.sup..about. 1.ANG. FWHM (full width half maximum)) and an irradiance of less than 15% of the continuum at line center.
There are a number of possible methods to generate a high power solid state source at 486 nm beginning with a neodymium laser operating at .about.1.06 micron. These methods generally involve nonlinear frequency conversion techniques which must be tunable at some stage in order to exactly match the required wavelength. The technique of optical parametric oscillation (OPO) is such a technique. This method requires a high power pump source which can be made to generate two additional waves of lower frequency due to nonlinear polarization in a suitable crystal. These waves are generally known as the signal and idler waves. The frequency and wave vector of the pump wave must equal the sum of the frequencies and wave vectors of the signal and idler waves. This requirement is known as phase-matching and is only possible in certain crystal for certain sets of frequencies and orientations of that crystal.
Present methods of generating 486 nm using the optical parametric oscillator (OPO) technique involve phase-matching by angle tuning of the nonlinear crystal. Such a technique has been described using 355 nm from a frequency tripled Nd:YAG laser to pump a BaB.sub.2 O.sub.4 crystal oriented .about.28 degrees to the c-axis; see the article by Y. X. Fan, R. C. Eckardt, R. L. Byer, J. Nolting, and R. Wallenstein, Appl. Phys. Lett, vol. 53, p. 2014 (1988). The technique of angle tuned phase-matching has a number of drawbacks. In order to control the precise wavelength of the generated output, the pump source is required to have a low beam divergence since the phase-matched signal wavelength is related to the pumping angle. Also, because of double refraction, the pump beam travels at a small angle to at least one of the generated beams in the crystal which leads to a reduction in their spatial overlap. This leads to decreased efficiency for converting the pump energy to signal energy. This effect is sometimes known as beam walk-off. Both of these effects often lead to practical imitations in OPO devices and can prevent scaling to larger sizes and higher power.
The nonlinear lithium borate crystal LiB.sub.3 O.sub.5 (LBO), which was recently developed in China, has received considerable attention for frequency mixing, as reported by C. Chen, Y. Wu, A. Jiang, B. Wu, G. You, R. Li, and S. Lin, J. Opt. Soc. Am. vol. B 6, p. 616 (1989); B. Wu, N. Chen, C. Chen, D. Deng, and Z. Xu, Opt. Lett, vol. 14, p. 1080 (1989); J. T. Lin, J. L. Montgomery, J. R. DeSalvo, and A. M. Horner, in Digest of Topical Meeting on Advanced Solid State Lasers, Optical Society of America, Washington, DC (1990), pp. 64-66; J. T. Lin, J. R. DeSalvo, J. L. Montgomery, and K. Kato, in Conference on Lasers and Electro-Optics, Vol. 7 of OSA 1990 Technical Digest Series, Optical Society of America, Washington, DC (1990), pp. 280-281; T. Ukachi, R. J. Lane, W. R. Bosenberg, and C. L. Tang, Appl. Phys. Lett., vol. 57, p. 980 (1990); and S. Lin, Z. Sun, B. Wu, and C. Che, J. Appl. Phys. vol. 67, p. 634 (1990); and parametric generation applications, see the article by M. Ebrahimzadeh, G. Robertson, M. H. Dunn, and A. J. Henderson, in Conference on Lasers and Electro-Optics, Vol. 7 of OSA 1990 Technical Digest Series, Optical Society of America, Washington, DC (1990), pp. 659-660. Lithium borate has a number of advantages compared with BaB.sub.2 O.sub.4 (BBO), a similar crystal also developed in China. The transmission extends further into the UV, it is not hygroscopic, and optical damage thresholds are reported to be even higher than for BBO. LBO is a biaxial crystal in the symmetry class mm2. The principal axes of the optical indicatrix, with the convention n.sub.2 &gt;n.sub.y &gt;n.sub.x, are related to the piezoelectric axes, which are used for the nonlinear response matrix, d.sub.ij, by (x,y,z).fwdarw.(a,c,b). LBO has a more complex phase-matching geometry than the uniaxial material BBO. Second-harmonic generation of 1.06 .mu.m is possible with both type I and type II orientations at 11.degree. from the x axis in the xy plane and 15 from the z axis in the zy plane, respectively; note the article by C. Chen et al. cited above. In accordance with this inventive concept, when the phase-matching condition is this near one of the principal axes, temperature tuning might form the basis to achieve noncritical phase-matching along that axis. An appropriate geometry such as disclosed in the disclosure of this inventive concept permits a significantly larger angular acceptance for the pump wave and eliminates the problem of double refraction. The angular acceptance is one constraint that often limits phase-matching path lengths and conversion efficiencies in practice. Noncritical type I phase-matched second-harmonic generation was recently demonstrated for fundamental wavelengths from 1025 to 1253 nm over the temperature range from 190.degree. C. to -3.degree. C., note the article by Ukachi et al. referenced above.
Thus, a continuing need exists in the state of the art for a parametric generation from temperature-tuned lithium borate at wavelengths of the Fraunhofer lines including H-Beta to assure increased transmission in seawater with reduced interference from background solar noise.